This invention relates to a ground anchor which is suitable for use in the reinforcement of rock.
As used herein “rock” includes rock strata, a cementitious body or similar hard material.
The provision of support in an underground mining excavation in a cost effective manner is of paramount importance.
Support structures such as hydraulically or mechanically extensible steel jacks, elongate wooden supports, mat packs, mechanically actuated or grouted rock bolts or cable anchors, and bags or tubes which are filled with a settable material, have all been used to provide support.
In narrower excavations mechanical ground anchors have not found widespread acceptance because of space limitations. It is difficult to drill vertical holes, up to two meters long, for steel anchors in a confined space. Reliance must be placed on extension drilling techniques with coupling rods. The installation of steel anchors is also problematic. A steel anchor should have a length which is about twice the height of the stope which is to be supported and must therefore be constructed from several short sections which are bolted together using extension sleeves at the time of installation. A polyester resin is commonly used to anchor a steel shank in a hole. The volume of resin which is needed to fill an annular space in a hole, around a bolt shank, can be high and the resin is expensive. Moreover if the quantity of resin is large then the bonding strength of the resin is effectively reduced and the steel anchor cannot carry its designed load. It is also difficult to assess the quality of the installation because the shank must be rotated, at the time of installation, to break the resin capsules and to mix the resin. Inadequate or excessive rotation adversely affects the shear strength of the resin.
A nut which is engaged with a protruding threaded end of the shank is tightened against a face plate which is engaged with the shank and which bears against the rock face. The nut and protruding end of the shank remain exposed. This is undesirable because the protruding components can severely restrict movement of men and machinery in a shallow excavation.
It is an object of the present invention to provide various components of a rock anchor which can be used alone, or in combination, to address some or all of the aforementioned problems. The invention is described hereinafter with particular reference to an anchor used in a horizontal narrow reef underground support application but this is exemplary only and is non-limiting.